1. Field of the Invention
The present invention relates in general to the control of a brushless DC (BLDC) motor. More particularly, the present invention relates to control a sensorless BLDC motor without Hall sensors.
2. Description of Related Art
FIG. 1 shows a schematic diagram for a BLDC motor drive. Such a motor usually comprises three phases A, B and C. The three phases A, B and C may be connected in a star configuration having a common node N as shown in FIG. 1, or in a delta configuration. For each phase, switch pairs XSA, XGA; XSB, XGB; XSC, XGC connect the free ends of the phases A, B, C to supply a power source Vs and a ground GND, respectively. The switches XSA, XGA, XSB, XGB, XSC, XGC are typically power transistors. Reverse biased diodes DSA, DGA; DSB, DGB; DSC, DGC are connected in parallel to the corresponding switches XSA, XGA, XSB, XGB, XSC, XGC.
The BLDC motor is controlled by detecting the position of the rotor, then a current is applied to the stator according to the detected position. Three Hall sensors are widely used as a position sensor for the BLDCM. However, the Hall sensors themselves increase the motor system size and manufacturing costs. Many position sensorless BLDCM drivers that detect the rotor position from the back electromotive force (BEMF) generated in the phases, have been introduced.
FIG. 2 shows a relationship between the BEMFs and armature currents of stator windings. The motor is controlled through six steps s1, s2, s3, s4, s5 and s6. In each step, the current is conducted in only two phases, and the third phase is floating. The BEMF can be monitored from the terminal voltage of the floating phase. The commutation events occur at 30° delayed from the corresponding zero-crossing points (ZCP) of the BEMF waveforms.
U.S. Pat. No. 4,654,566 describes such a motor control system, which is based on sensing the difference between the virtual neutral potential and the voltage at the floating terminal. For the motor driven by a main power, the supply voltage Vs may be 300V respective to ground GND. The monitored BEMF must be attenuated into an allowable range for the sensing circuit. Therefore, the method tends to have a narrow speed range and a poor signal to noise ratio.
To achieve wide speed range, especially at a lower speed, U.S. Pat. No. 5,859,520 describes a direct BEMF detection circuit E referring to FIG. 3. With the clamping circuitry C, there is no attenuation of the monitored BEMF, which has good resolution even at low speed operation. However, the resistor R for limiting the injected current has a high value, such as 100 kΩ. The BEMF zero crossing detection is not correct sometimes when the PWM duty cycle is high. This is caused by the large time constant of the current limit resistor and the parasitic capacitance inside the microcontroller.